I. Field of the Invention
The present invention relates to spread spectrum communication systems, such as wireless data or telephone systems, and satellite communication systems. More particularly, the invention relates to a method and apparatus for generating, identifying, and acquiring spread spectrum communication signals using layered or overlayed PN spreading and identifier codes having differing periods or chip rates.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users, such as code division multiple access (CDMA) spread spectrum techniques. CDMA techniques in multiple access communication systems are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, under the title xe2x80x9cMethod And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,xe2x80x9d which are both assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which a large number of generally mobile or remote system users or subscribers use transceivers to communicate with other system users or desired signal recipients, such as through a connected public telephone switching network. The system users communicate through gateways and satellites, or terrestrial base stations (also referred to as cell-sites or cells) using CDMA spread spectrum communication signals.
In a typical spread-spectrum communication system, one or more sets or pairs of preselected pseudorandom noise (PN) code sequences are used to modulate or xe2x80x98spreadxe2x80x99 user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the underlying data signal. In the base station- or gateway-to-user communication link, also referred to as the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or between signals of different beams, satellites, or gateways, as well as between multipath signals.
These codes are typically shared by all communication signals within a given cell or beam, and time shifted or offset between adjacent beams or cells to create different spreading codes. The time offsets provide unique beam identifiers which are useful for beam-to-beam handoff and for determining signal timing relative to basic communication system timing.
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between signals intended for different users within a cell or between user signals transmitted within a satellite beam, or sub-beam, on a forward link. That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique xe2x80x98coveringxe2x80x99 or xe2x80x98channelizingxe2x80x99 orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. In this arrangement, each Walsh function of 64 or 128 chips is typically referred to as a Walsh symbol.
PN code based modulation techniques used in CDMA signal processing allow spectrally similar communication signals to be quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished from each other, provided path length differential causes relative propagation delays in excess of the PN code chip period. If a PN chip rate of say approximately 1.22 MHz is used, a spread spectrum communication system can distinguish or discriminate between signals or signal paths differing by more than one microsecond in path delay or time of arrival.
Wideband CDMA techniques permit problems such as multipath fading to be more readily overcome and provide a relatively high signal gain. However, some form of signal diversity is also generally provided to further reduce the deleterious effects of fading and additional problems associated with acquiring and demodulating signals in the presence of relative user and satellite or source movement within a communication system. Such movement along with large distances causes substantial dynamic changes in path lengths. Generally, three types of diversity are used in spread spectrum communication systems, including time, frequency, and space diversity. Time diversity is obtainable using error correction coding or simple repetition and time interleaving of signal components, and a form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Space diversity is provided using multiple signal paths, typically, through different antennas or communication signal beams.
Typical CDMA spread spectrum communication systems contemplate the use of coherent modulation and demodulation techniques for forward link user terminal communications. In communication systems using this approach, a xe2x80x98pilotxe2x80x99 signal (or other known signal) can be used as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station or gateway throughout a given region of coverage. A single pilot is typically transmitted by each gateway or base station for each frequency used, typically referred to as a CDMA channel, an FDM channel, or as a sub-beam in some systems. This pilot is shared by all users using that CDMA channel, from a common source. Generally, sectors each have their own distinct pilot signals while satellite systems transfer a pilot within each satellite beam, or frequency or sub-beam, which originates with gateways using the satellite. This provides signals that can be readily distinguished from each other, also distinguishing between beams and cells while providing simplified signal acquisition.
Pilot signals are employed by user terminals to obtain initial system synchronization, and provide robust time, frequency, and phase tracking of transmitted signals and a channel gain reference. Phase information obtained from a pilot signal is used as a phase reference for coherent demodulation of communication system or user information signals. Since pilot signals do not generally involve data modulation, they essentially consist of the PN spreading codes which are modulated onto a carrier frequency. Sometimes, the PN spreading codes are referred to as pilot code sequences. The PN spreading codes are generally time shifted with respect to each other to achieve distinguishable pilot signals.
Pilot signals are generally used to gauge relative signal or beam strength for received communication signals. In many systems, pilot signals are also generally transmitted at a higher power level than typical traffic or other data signals to provide a greater signal-to-noise ratio and interference margin. This higher power level also enables an initial acquisition search for a pilot signal to be accomplished at high speed while providing for very accurate tracking of the pilot carrier phase using relatively wide bandwidth, and lower cost, phase tracking circuits.
As part of the process of establishing a communication link, the user terminal employs a receiver referred to as a xe2x80x98searcher receiverxe2x80x99, or simply xe2x80x98searcherxe2x80x99, to synchronize with the pilot phase and PN spreading code timing in the presence of unknown carrier frequency offsets. Several techniques and devices have been used to provide this searcher function. One such technique is disclosed in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDiversity Receiver In A CDMA Cellular Telephone System,xe2x80x9d issued Apr. 28, 1992, which is assigned to the assignee of the present invention, and incorporated herein by reference.
One of the problems associated with pilot acquisition/synchronization and signal demodulation processes is the amount of time required for users to acquire the pilot signals. More accurately, it is the amount of time required to acquire the phase or timing of the PN spreading codes used in generating pilot signals, for use in demodulating other communication signals.
In terrestrial repeater based systems, such as land based wireless cellular telephone services, a relatively long PN code sequence of 32,687 chips in length is used, which is docked at a chipping rate on the order of 1.2288 mega-chips per second (Mcps). This length is useful in differentiating signals in a system having a large number of closely spaced cells. Since such wireless systems have consistently strong pilot signals, acquisition times can remain short for this length. That is, with robust pilot signals, and little or no Doppler frequency shifting, or similar effects, the time required to select and verify a correct phase, or signal timing, is still relatively short. However, for satellite based systems, Doppler effects on the frequency, and degradation of pilot signal power along with lower power pilot signals, generally results in longer times for acquiring and verifying pilot signal timing.
Therefore, shorter PN spreading codes have been contemplated in order to help substantially shorten the overall searching or acquisition time in view of the lengthened time taken for testing hypotheses, verification, and so forth. In this type of communication environment, PN codes on the order of 1024 chips in length have been contemplated, which results in a code length of about 833 xcexcsec., at the chip rate mentioned above. Many systems package information bearing channels into blocks of bits, or xe2x80x9cframes,xe2x80x9d where frame synchronization is required before the bits can be used. The exact meaning or subsequent processing of the information bits is a function of location within the frames. Such data frames are typically 20 to 80 msec. in length, which creates problems in determining proper frame timing when working with the much shorter PN codes. A short PN code by itself leaves many unresolved hypotheses of frame timing. The correct frame timing can only be found by trial and error of the different hypotheses. This uncertainty in frame timing delays the acquisition of the information channels or signals.
Unfortunately, the path delays for signal transfers from gateways-to-satellites and satellites-to-users or transceivers also create a major problem for shortening the PN codes. The distances involved, even for low Earth orbits, impose significant path delays on signals, which can vary widely depending on satellite orbital positions. This results in the signal time offsets for different satellites or signal sources being significantly shifted relative to each other, so that signals otherwise offset from each other begin to be aligned, which prevents proper signal differentiation. That is, signals are affected by a dynamic range of path delays on the order of 7 msec. which means they are no longer adequately separated in time and cannot be properly distinguished as to beams or signal sources. The obvious solution of lengthening the PN spreading codes by even a small amount re-introduces undesirable time delays in signal acquisition.
Therefore, what is needed is a new technique for spreading forward link signals so that receivers can still acquire phase and beam identification information used for signal demodulation over short time intervals while compensating for relatively high signal delay paths and lower power pilot signals associated with satellites moving relative to signal recipients.
In view of the above and other problems found in the art relative to acquiring and processing communication signals in spread spectrum communication systems, one purpose of the present invention is to improve signal acquisition.
One advantage of the present invention is that it provides for the use of short PN sequences for signal acquisition while maintaining signal differentiation for identification, and improving synchronization to information channel timing.
These and other purposes, advantages, and objects of the invention are realized in a method and apparatus for spreading signals in a spread spectrum communication system in which digital information signals are bandwidth spread using a preselected pseudorandom noise (PN) spreading code to produce spread spectrum modulation signals. An exemplary communication system is a wireless data or telephone system that uses multiple satellite repeaters to receive communication signals from gateway type base stations and transfer them to one or more of a plurality of mobile or portable stations having receivers. Information signals in such systems are generally converted from analog to digital form, as necessary, and then interleaved and encoded for error detection and correction purposes before being transferred to system users. The encoded signals may be combined with one or more orthogonal functions to provide channelization of the information signals.
In a preferred embodiment, a first PN spreading code is generated with a preselected first code length and first period or periodicity. This code is referred to as an inner code. A second PN code sequence is produced having a second predetermined code length, and a period substantially longer than the first. This code is referred to as an outer code. The PN codes can be generated using first and second PN generators, respectively. In some systems PN code generation devices or circuits may be time shared in generating some of the codes or sequences. The update or generation rate or xe2x80x9cchipping ratexe2x80x9d for the second PN code or code generator is significantly less than the update or generation rate of the first.
Typically, the first PN spreading code is input to a first spreading means or element where it is used to spread the information signals to be transmitted, resulting in the production of first spread spectrum signals. The resulting first spread spectrum signals are input to a second spreading element where they are combined with the second PN code sequence to produce second spread spectrum signals. Typically, multipliers are used to combine the PN codes and signals at each step. The resulting spread spectrum signals can be transferred to transmission circuits for modulating onto a carrier signal, followed by transmission by the communication system to one or more system users.
However, in further aspects of the invention the second PN code is combined with the information signals first and then the first PN code is used for spreading the resulting signals. Alternatively, the two codes are combined to produce a unique spreading code that is essentially an outer code modified inner code, which is then used to spread the information signals.
In an exemplary spread spectrum system, information signals are applied equally to an In-Phase channel and a Quadrature-Phase channel and the first spreading element uses a PN code generator to produce an In-Phase PN chip code for one channel using a first polynomial function, and a second PN code generator to produce a Quadrature-Phase PN chip code for the other channel using a second, different, polynomial function. The second spreading element uses a third PN code generator to produce a third PN chip code using yet another polynomial function
The entire first PN spreading code period is equal to one chip period for the second PN code, and the respective periods of the PN codes are synchronized to begin at the same time. These codes can be implemented, for example, as pre-selected portions of m-sequence PN codes, or augmented length maximal-length linear sequence PN codes. The longer total code period PN code or code sequence forms an xe2x80x98outerxe2x80x99 code for which system timing is easier to acquire, while the shorter period PN spreading code forms an xe2x80x98innerxe2x80x99 code that maintains a desirable level of signal non-interference. The overall affect is to provide improved signal identification and synchronization to signal timing, while maintaining reasonably fast signal acquisition.
A code sequence found useful for the second PN code of the invention when a first PN code of length 1024 is used, is 288 chips long and has chip values beginning with the series or set xe2x88x921 xe2x88x921 1 xe2x88x921 1 xe2x88x921 xe2x88x921 1 1 xe2x88x921 xe2x88x921 xe2x88x921 1 xe2x88x921 1 1 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 1 xe2x88x921 1, and ending with all remaining chips being 1. Alternatively, a code found useful is xe2x88x921 xe2x88x921 xe2x88x921 1 1 xe2x88x921 1 xe2x88x921 xe2x88x921 1 xe2x88x921 1 1 1 xe2x88x921 1 1 1 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 1 1 1 . . . 1. Another code found useful is generated using the characteristic polynomial Q(z)=1+z3+z4+z6+z9, and then using a 288 chip sequence.
In further aspects of the invention, spreading elements can be implemented by storing pre-selected PN codes in data storage means or memory elements, such as ROM or RAM circuits. The codes are then retrieved and provided as inputs to multipliers that also receive corresponding information or spread signals as inputs.
The second PN code can also be differentially encoded, to reduce requirements for phase coherence, by passing the retrieved code through a one-chip delay element and inputting it to another multiplier which also receives undelayed code. The multiplier forms a product between the delayed and undelayed PN code, and provides the product as a differentially encoded output. Alternatively, the data storage could contain a differentially encoded version of the second PN code sequence.
On the reception side of the communication system, the timing of such multi-layered spread spectrum communication signals is acquired using a receiver in which communication signals are first demodulated to remove the carrier and then despread. A despreader or despreading means combines the received spread spectrum signal with the second or inner PN spreading code to produce a first level or intermediate despread signal. An accumulator is used to accumulate this despread signal over the period of the second PN code or a chip period of the first code, and differentially detects the phase shift between consecutive accumulated signals, or decodes the accumulated signals. The detected signal is subjected to a matched filtering process, and the results compared to a preselected threshold value. In addition, the threshold value used in this comparison can be preselected or produced by determining an average value for the magnitude of the detected signal over the first PN code period. This value can be scaled appropriately.